Electrospray interfaces are used to deliver charged ions in the gas phase to a mass analyzer, generally a mass spectrometer. In an electrospray interface for a mass spectrometer, a charged capillary tube delivers a stream of liquid solvent to a discharge point at one side of an ionization chamber. An oppositely charged plate forms an opposite side of the ionization chamber. The stream of liquid solvent forms a cone at the tip of the capillary tube, with the tip of the cone extending away from the capillary tube, as charged particles in the liquid at the tip of the capillary tube are attracted towards the charged plate. Charged droplets of opposite polarity to the polarity of the capillary tube separate from the liquid cone and drift downfield towards the charged plate. As the charged droplets drift towards the charged plate, the liquid evaporates, thus forming gas phase ions. An orifice in the charged plate admits the gas phase ions into the ion analyzer portion of the mass spectrometer where they are detected as an observable analyte signal. Further description of the ion forming process using conventional electrospray interfaces may be found in "Electrospray-Ion Spray: A Comparison of Mechanisms and Performance", Ikonomou, M. G., Blades, A. T. and Kebarle, P., Anal. Chem. 1991, 63, 1989-1998.
A useful discussion of prior art methods and apparatus for extracting ions from a liquid solvent stream as applied to the field of mass spectrometry is found in U.S. Pat. No. 4,935,624 to Henion et al, col. 1, line 24, to col. 18, line 18. That patent describes a thermally assisted electrospray interface (TAESI) that is intended to overcome then existing prior art problems.
As described in the Henion et al patent, it has been found advantageous to heat the liquid solvent stream in the capillary tube, and this has found utility in the treatment of liquid solvent streams having a moderate percentage of water.
In general, however, conventional electrospray mass spectrometry of neat aqueous (pure water) solutions is much less successful when compared to electrospray involving solvents like methanol, ethanol and acetonitrile. When pure water is used as a solvent, the analyte signal observed is relatively very unstable and the observed ion intensity at a given analyte concentration is low. Yet electrospray of aqueous solutions is very desirable particularly where analysis of proteins is involved, since water is a good solvent for proteins, such as in reverse phase high protonation liquid chromatography (HPLC) and capillary electrophoresis (Ewing, A. G.; Wallingford, R. A.; Olefirowicz, T. M., Anal. Chem. 1989, 61,292.)
R. D. Smith et al. (Anal. Chem. 1990, 62,882.) found a method with which they could electrospray aqueous solutions. In their "Liquid Sheath Electrode" they combine, at the capillary tip, an outer liquid sheath of methanol with the inner aqueous flow emerging from the capillary. Thus, the solvent sprayed is actually a water-methanol mixture which is amenable to electrospray. While the method appears to work well, it still remains desirable to develop a method with which aqueous solutions can be electrosprayed without premixing with methanol.
One of the factors leading to poor performance for water as solvent is well understood, but, nonetheless, the solution to the problem has not previously been found. P. P. H. Smith (IEEE Trans. Ind. Appl. 1986, 1A-22,527) has shown that the electric field, E.sub.on, required for the onset of liquid instability at the capillary tip and thus for the onset of electrospray is given by eq. 1. EQU E.sub.on =(2.gamma.cos.theta./.epsilon..sub.o r.sub.c).sup.1/2( 1)
where .gamma. is the surface tension, .theta.=49.degree. is the half angle of the liquid cone (Taylor cone) at the capillary tip and .epsilon..sub.o and r.sub.c are the permittivity of vacuum and the radius of the capillary. Since water has a surface tension (.gamma.=0.073 N m.sup.-2) which is more than three times higher than that of methanol (.gamma.=0.023 N m.sup.-2), the E.sub.on for water is close to two times higher. Generally, for a stable spray one needs a potential that is some 200-300V above the onset potential and for water the potential that would have led to stable spray causes electrical breakdown in the ambient gas, air (Smith, D. P. H. above cited and Ikonomou, M. G.; Blades, A. T.; Kebarle, P., J. Am. Soc. Mass Spectrom. 1991, 2, 497.). Gas phase ions are then formed not only by the electrospray process but also by gas phase ionization due to corona discharge. While it is generally easy to distinguish with the mass spectrometer between electrospray ions and discharge ions, the presence of electric discharge, which can be intermittent in this voltage range, leads to an instability of the electrospray derived analyte signal. The analyte signal is also strongly depressed when a discharge current approaching 0.8 .mu.A or higher is present. This appears from work previously done by the inventors (Ikonomou, M. G.; Blades, A. T.; Kebarle, P., J. Am. Soco Mass Spectrom. 1991, 2, 497).
The discharge can be suppressed by the application of SF.sub.6 gas flow around the electrospray capillary tip. Suppression of the discharge with SF.sub.6 was found to lead to stable analyte signals and an increase of the analyte ion intensity. However, the sensitivity still remained lower by a factor of about 4 relative to that observed with methanol, even though the total capillary current l.sub.c was essentially the same when water or methanol were used as solvents.
From their analysis of the problem, the inventors have determined that the lower yield of gas phase ions from the charged droplets for water relative to methanol could be due to various factors such as:
(a) Larger droplets for a given charge could be formed when water was sprayed. The initial size of the droplets is one of the most important parameters. The droplets should not only be close to the Rayleigh limit but also very small. Under these conditions droplets small enough to lead to ion evaporation into the gas phase may be generated only after a few Rayleigh explosions.
(b) For droplets of equal radius and equal charge, the rate of solvent evaporation and thus the time required to reduce the droplet size to where gas phase ion emission occurs, could be lower for water.
(c) For droplets of sufficiently small size, so that gas phase ion emission can occur, ion evaporation could be slower out of water relative to methanol droplets of same size and charge.
The recently proposed equation of due to Fernandez de la Mora (Fernandez de la Mora, J.; Hering, S.; Rau, N.;McMurry, P., J. Aerosol. Sci. 1989, 21, 169) with which the relative size of droplets can be predicted (eqtn. 2) EQU d.varies.(pQ.sup.2 /.gamma.).sup.1/3 ( 2)
where d is the droplet diameter, p the density of the solvent and Q the flow rate, provides an answer to question (a). According to eq. 2 the ratio of droplet size at the same flow rate should be: ##EQU1## Thus, according to eq. 2 MeOH should lead to somewhat larger droplets and this means that factor (a) could not be responsible for the lower ion yield from water.
No simple equation is known to the inventors regarding (b), particularly for droplets which are not stationary in the ambient gas. Expressions dealing with evaporation rates and key references can be found in Davis et al. (AIChE Journal 1988, 34, 1310). The most important single parameter appears to be the vapour pressure of the solvent. The vapour pressures at 20.degree. C. are: p(MeOH).apprxeq.100 torr, p(HOH).apprxeq.20 torr. On this basis one would expect that MeOH droplets initially of equal size will reduce their size much more rapidly than water droplets.
In previous work by the inventors (the above cited work and also Ikonomou, M. G.; Blades, A. T.; Kebarle, P., Anal. Chem. 1991, 63, 1989-1998), question (c) is examined on the basis of the Iribarne model (Wong, S. F.; Meng, C. K.; Fenn, J. B., J. Phys. Chem. 1988, 92,546) for very small droplets, r&lt;10.sup.-2 .mu.m, and came to the conclusion that the rate of ion evaporation will be higher for methanol relative to water, when droplets of the same charge and radius are compared.
It appears therefore, that the factors (b) and (c) are responsible for the lower gas phase ion yield out of water droplets. On that basis one might expect that heating of the droplets will lead to better performance when water is used. However, as the Henion device has shown, heating of the droplets is not sufficient when pure water is used as the solvent.
It is known in itself to use heat to improve electrospray performance, as with the Henion patent and also as reported by Smith, R. D.; Barniaga, C. J. and Udseth, H. R., Anal. Chem. 1988, 60, 1948-52. Chowdhury, S. K.; Katta, V. and Chait, B. T., Rapid Commun. Mass Spectrom. 1990, 4, 81. and Reid, N.; Buckley, J. A.; French, J. B.; Poon, C. C., Adv. Mass Spectrom. 1979, 8B, 1843. However, the use of heat in these latter three articles was not introduced specifically for electrospray of water. Fenn et al. (Wong, S. F.; Meng, C. K.; Fenn, J. B., J. Phys. Chem. 1988, 92,546) have reported the use of drying gas, N.sub.2, heated to 330.degree.-350K. Comparative experiments demonstrating the utility of this procedure were not reported.
R. D. Smith et al. (Anal. Chem. 1988, 60, 1948-52) have used heated nitrogen (interface) gas at 70.degree. C. flowing in a counter current to the spray, for the desolvation of the gas phase ions. The purpose of these experiments was different from that of the present invention.
Chait and coworkers (Chowdhury, S. K.; Katta, V. and Chait, B. T., Rapid Commun. Mass Spectrom. 1990, 4, 81.) used a heated capillary tube (80.degree.-100.degree. C.) to transfer electrospray from atmospheric pressure to the vacuum system of the mass spectrometer. The purpose was to desolvate the droplets and gas phase ions. The solvent used was a 50:50 methanol-water mixture. A capillary temperature of 85.degree. C. was found to maximize the protonated peptide signals. Experiments with other solvent mixtures and neat water were not reported.
The inventors have developed a new, heated, interface having surprising utility particularly with pure water solvents but also with other solvents. The present heated electrospray interface is very different from those used in previous work known to the inventors. Furthermore, the present heated electrospray provides outstanding performance for electrospray of aqueous solutions as shown by comparisons with electrospray obtained without heating and with other solvents (methanol).
The inventors have found that by the surprising expedient of using an entraining flow of gas about the spray of solvent, the sensitivity of the electrospray interface with pure water as the solvent may be greatly increased.
Thus one aspect of the invention provides a heated capillary source or tube having heated gas flow channels disposed about the heated capillary source such that the channels supply a flow of gas that heats and entrains the liquid solvent. The liquid solvent should have a temperature within the capillary source that is close to its boiling point. Preferably, the flow of gas is atmospheric air at atmospheric pressure, and has a temperature greater than the boiling point of the liquid solvent. A sleeve or other gas flow entraining means is useful to ensure that the flow of gas is concentric to the liquid solvent spray.
In one aspect of the method of the invention, ion production from a liquid solvent spray of an electrospray interface is enhanced by heating and entraining the liquid solvent spray with a flow of gas that surrounds and entrains the liquid solvent spray. The liquid solvent is preferably heated within the capillary to a point where it is near the boiling point of the liquid solvent and the gas has a temperature that is higher than this, preferably at least 20.degree. C. higher for acetonitrile or methanol based solvents and 50.degree. C. for pure water.
An electrospray interface in which the electrospray capillary is heated and the liquid solvent sprayed from the capillary is heated by a hot air stream which is coaxial and in the same direction as the spray, provides excellent performance for solvents in which water is the major component, including 100% water. This meets a very much needed requirement for HPLC and capillary electrophoresis applications of electrospray.
The coaxial and codirectional gas flow improves, by convection, the transport of droplets and ions to the sampling orifice. The near boiling point temperature of the solution at the capillary leads to a lowering of the surface tension of water and thus a lowering of the capillary voltage required for the onset of the spray. The initial high temperature of the droplets when formed and the subsequent heat transfer from the heated air stream facilitate the evaporation of water from the droplets and lead to rapid shrinkage of the droplets to the Rayleigh limit. Droplets that have reached the ion evaporation radius r&lt;0.01 .mu.m produce gas phase ions more efficiently when hot.
These and further aspects of the invention are described in more detail in the next two sections, and claimed in the claims that follow.